Editor UI Cleanup (#3676)

[[PR Description]]

TODO: 
- [x] Style fold marker
- [x] Editor - Code action icon too large, should use muted color,
should have tooltip
- [ ] Style inline assistant
    - Prompt text gets cut off veritcally
- [x] Line height on inline assist editor isn't tall enough
- Nate: This is my fault due to the line height (relative: 1) we are
setting on non-buffer editors. I'll look into this.
- [ ] Style diagnostic hover (right now it's an opaque box on some
themes)
    - Nate: This needs status tints, we are working on it
- [ ] Hovering over a symbol shows a dialogue that appears underneath
the scroll-bar

Release Notes:

- N/A

crates/editor2/src/element.rs

@@ -939,11 +939,11 @@ impl EditorElement {
                                     cx,
                                     |fold_element_state, cx| {
                                         if fold_element_state.is_active() {
-                                            gpui::blue()
+                                            cx.theme().colors().ghost_element_active
                                         } else if fold_bounds.contains(&cx.mouse_position()) {
-                                            gpui::black()
+                                            cx.theme().colors().ghost_element_hover
                                         } else {
-                                            gpui::red()
+                                            cx.theme().colors().ghost_element_background
                                         }
                                     },
                                 )